The present invention generally relates to processing of crystals. More particularly, the present invention provides a method for obtaining a gallium-containing nitride crystal with a release layer, but there can be others. In other embodiments, the present invention provides a method of manufacture of a high quality epitaxial gallium containing crystal with a release layer, but it would be recognized that other crystals and materials can also be processed. Such crystals and materials include, but are not limited to, GaN, AlN, InN, InGaN, AlGaN, and AlInGaN, and others for manufacture of bulk or patterned substrates. Such bulk or patterned substrates can be used for a variety of applications including optoelectronic devices, lasers, light emitting diodes, solar cells, photoelectrochemical water splitting and hydrogen generation, photodetectors, integrated circuits, and transistors, among other devices.
Gallium nitride (GaN) based optoelectronic and electronic devices are of tremendous commercial importance. However, the quality and reliability of these devices is compromised by very high defect levels, particularly threading dislocations, grain boundaries, and strain in semiconductor layers of the devices. Dislocations can arise from lattice mismatch of GaN based semiconductor layers to a non-GaN substrate such as sapphire or silicon carbide. Grain boundaries can arise from the coalescence fronts of epitaxially-overgrown layers. Additional defects can arise from thermal expansion mismatch, impurities, and tilt boundaries, depending on the details of the growth method of the layers.
The presence of defects has a deleterious effect on epitaxially-grown layers, compromising electronic device performance and requiring complex, tedious fabrication steps to reduce the concentration and/or impact of the defects. While a substantial number of growth methods for gallium nitride crystals have been proposed, the methods to date still merit improvement.
For some applications, it is desirable to remove most or all of the substrate from the GaN-based device. For example, it may be possible to remove heat more effectively from an active layer using flip-chip bonding. It may also be possible to extract light more efficiently from a light-emitting diode (LED) when the substrate is removed. In the case of an ultraviolet light emitting diode (UV-LED), with an emission wavelength shorter than 365 nanometers, a low-defect bulk GaN substrate would absorb much of the emitted light.
Several authors have demonstrated substrate-release techniques to separate GaN-based epitaxial layer from non-GaN substrates. For example, Fujii et al. [Applied Physics Letters 84, 855 (2004) fabricated LEDs on a sapphire substrate, laser-lifted-off the sapphire substrate, and then roughened the newly exposed (0 0 0-1) GaN surface to improve light extraction. Similarly, Kawasaki et al. [Applied Physics Letters 89, 261114 (2006) fabricated a UV-LED emitting at 322 nm by deposition of AlGaN active layers on sapphire, followed by laser lift-off of the sapphire. In a different approach, Ha et al. [IEEE Photonics Technology Letters 20, 175 (2008)] fabricated a vertical LED by depositing GaN-based epitaxial device layers on a CrN layer which in turn was deposited on a sapphire substrate, followed by chemical etching of the CrN layer. Oshima et al. [Physica Status Solidi (a) 194, 554 (2002)] fabricated thick, removable GaN layers using a TiN release layer, in which pores or voids were generated by etching in H2. However, these methods were developed for GaN layers on non-GaN substrates and, as a consequence, the epitaxial device layers have a relatively high dislocation density. It is possible to reduce the surface dislocation density by methods that are known in the art, such as epitaxial lateral overgrowth, but these methods generally are not able to produce layers with a dislocation density below about 105 cm−2 over the entire surface. In addition, the presence of voids or porosity may be deleterious to material quality in relatively thin epitaxial layers.
Significant progress has been made in the growth of bulk gallium nitride crystals and wafers with a low dislocation density, and GaN-based devices can be fabricated on these substrates rather than on sapphire. D'Evelyn et al. [U.S. Pat. No. 7,053,413, hereby incorporated by reference in its entirety] teach fabrication of a homoepitaxial LED on a bulk GaN substrate with a dislocation density below 104 cm−2, followed by removal of a portion of the substrate. However, the only means taught for removal of the portion of the substrate are lapping, polishing, chemical etching, plasma etching, and ion beam etching. These methods do not provide a natural endpoint, and it is therefore difficult to remove all but a few-micron-thick layer of uniform thickness, and are slow and expensive to perform.
What is needed is a means for providing a low dislocation-density substrate for homoepitaxial device manufacturing, where all but a few microns or less of the substrate can be removed or released accurately and cost effectively on a manufacturing scale.